Russian Patent Specification No. 627,039 relates to a device for discharging sheets from the top of a stack of sheets held therein. The device described therein has a fixed support which supports a holder for the sheets at a point situated substantially straight beneath the center of gravity of the stack of sheets received in the holder. The center of gravity of the holder is situated on that side of the center of gravity of the stack which is remote from the conveyor roller so that the stack is pressed against the conveyor roller with a substantially constant force. In this device, the exact direction in which the sheets are discharged from the stack depends on the thickness of the stack. The angle between the direction of discharge of the first sheet and the direction of discharge of the last sheet increases in proportion to the thickness of the stack of sheets.
If this device is coupled to a sheet-processing machine such as a copying machine or a laser printer, which has a fixed conveying nip for conveying a sheet discharged from the stack, the sheet must be deflected by a guide means to be able to reach the nip. If this conveying nip is formed with a guide plate extending between the stack and the separation nip so that the plate also serves to retain a second sheet possibly entrained with the top sheet of the stack which is being removed, then the plate must project relatively far above the holder. This is to ensure that any sheets of the stack which are entrained with the top sheet when the stack is tipped at a sharp angle, come into contact with the plate before reaching the conveying nip. The disadvantage of this device is that it becomes bulky, particularly when the device must accommodate a relatively thick stack of sheets such as when it is used as a feed system for receiving sheets in a relatively fast copying machine. Another disadvantage of this device is that there must also be a free space beneath the holder to enable it to tip about the fixed support point as the stack becomes thinner. This also makes the device bulky.
Japanese Patent Abstract Nos. 60-171936 (Vol. 10, No. 10, Jan. 16, 1986) and 60-12426 (Vol. 9, No. 128, June 4, 1985) as well as German patent application No. 3,309,985 all show devices for discharging sheets from the top of a stack of sheets disposed on a platform which is upwardly biased towards a fixed discharging roller by means of a spring. None of these devices deals with automatically adjusting the biasing force between the stack and the discharging roller so that it is independent of the type or weight of sheets disposed on the platform.
U.S. Pat. No. 4,350,328 also shows a device for discharging sheets from the top of a stack of sheets disposed on a platform which is upwardly biased by a spring towards a fixed discharging roller. In this device a member can be located intermediate the platform and the stack to augment the exerted biasing force. The disadvantage with this device is that the adjustment of the biasing force is left to the operator instead of being automatically adjusted. Failure by the operator to adjust the biasing force or error in setting the biasing force can result in disturbances during sheet discharge.
Thus, there is a need for a device for holding and discharging sheets from the top of a stack of sheets which does not have these disadvantages.